In conventional cellular wireless communications technologies, user equipment communicates with a base station by using an air interface and forwards user data to a destination node on a network by using the base station.
With development of mobile communications technologies, to better meet user requirements and improve efficiency of information exchange between user equipments, a mechanism of direct communication between user equipments, that is, device-to-device (D2D) communication, also referred to as sidelink communication, is introduced in a long term evolution (LTE) system. Different from the conventional cellular wireless communications technologies, data of user equipment does not need to be transmitted by using a base station, and is transmitted directly by using an air interface between user equipments, thereby reducing a network forwarding delay. In a D2D communication mode, a direct communication link between user equipments may be established under control or with assistance of a network device such as a base station. A typical application scenario of D2D communication is a public safety scenario, such as communication between fire fighters. A typical service type is a voice service.
As shown in FIG. 1, a D2D communications system includes a base station 101, user equipment (UE) 1, and UE 2. The base station 101 and the UE 1 may perform cellular wireless communication. In addition, the UE 1 and the UE 2 may perform D2D communication.
Currently, D2D communication supports group communication, that is, one-to-many communication, in which D2D user equipment may send D2D data to several user equipments in a communication group. As shown in FIG. 2, a base station 201 controls or assists UE 1, UE 2, UE 3, and UE 4 in establishment of D2D group communication. For example, a group 202 is a public safety group. The UE 1 may directly transmit data to the UE 2, the UE 3, and the UE 4 without using the base station.
Currently, D2D communication further supports unicast communication, that is, one-to-one communication, in which D2D user equipment directly sends D2D data to another user equipment.
However, because transmission resources are obtained by means of contention between user equipments, there may be a problem in an existing D2D communications system, for example, some data is not transmitted in a sufficiently timely and reliable manner because some user equipments fail to obtain resources by means of contention.
Because of the inherent disadvantage of the D2D communication manner in the existing wireless communications system, a new D2D data transmission method and device are urgently needed in the art, to resolve the problem that may exist in the existing D2D communications system, for example, some data is not transmitted in a sufficiently timely and reliable manner.